This invention relates to improvements in induction means or method of heating the edges of a tube formed from skelp and relates particularly to the overheating of edges prior to butt welding of the edges by compression.
Most of the presently operating hot mills preheat skelp uniformly in fossil-fueled furnaces and, as the skelp exits from the furnace, quickly form it into an open C-shape and then apply compressive force simultaneous with the introduction of an oxygen stream which provides the temperature necessary to weld the edges of the tube together. Preheat temperatures required by this process result in the generation of large quantities of scale which not only reduce the productive capability of the installation but provide for the potential of entrapment of scale within the weld and result in an inferior product.
The design of fossil fuel furnaces used to heat strip prior to welding is established by the throughput in tons per hour, the desired temperature, and the skelp width and thickness. Most often, the skelp will reverse its direction in the furnace a number of times to reduce the floor space required and, by making the furnace more compact, increase the thermal efficiency of the heating operation. The maximum rate of heat transfer is a function of the temperature difference between the hot gases and the cooler strip. If a strip temperature of 2400.degree. F. is desired, an overtemperature of the hot gases of 200.degree. F. may be required to keep the size of the furnace reasonable. At this temperature, the skelp oxidizes rapidly.
Other means for providing energy for overheating the edges of the skelp may be employed whereby the temperature of the strip as it exits the furnace can be reduced. This can result in many operating benefits. Various induction means have been proposed to overheat the edges to achieve reduction of scale loss and improvement of the quality of the weld. In U.S. Pat. No. 2,673,274 to Vaughan, an induction heating coil is proposed to overheat the edges while the skelp is flat. A preheat temperature of 1500.degree. is suggested. In a more recent U.S. Pat. No. 4,375,512 to Nishimoto, an alternative induction means is described which results in overheating the edges of the skelp. In that patent, a preheat temperature of 1832.degree. F. is suggested. In both these methods, because of the time required for the skelp to pass from the influence of the induction heating coil to the weld zone, the temperature of the edges decreases and it is hence necessary to heat a greater portion of the skelp to a higher temperature than would be required if the heating were applied just prior to the weld rolls. Various other methods have been proposed, such as that disclosed in U.S. Pat. No. 2,652,474 to Sorensen, utilizing induction heating to heat the edges of skelp that has been cold formed. This inductor configuration does not provide for equal heating of the opposing edges under usual operating conditions.